Science: So farewell, snowy owl: official - Twenty thousand amateur ornithologists have helped to produce a new census of Britain's breeding bird populations. Malcolm Smith reports on some of their discoveries

THEY almost certainly never sang in Berkeley Square but, just 15 or so miles from central London, nightingales can still be heard in the woods of northwest Kent. Of the 5,000 or so pairs still breeding in Britain, more than half are found in Kent, Sussex, Surrey, Hampshire and Suffolk. In the past 20 years this enigmatic bird of dense woodland has disappeared from swathes of wooded countryside stretching from Lincolnshire to west Dorset. Its range has been reduced by more than a quarter.

Ornithological statistics such as these can too easily be taken for granted. The amount of recording effort and data analysis required to provide even a simplified summary of the status and distribution of one species is enormous.

Consider, then, the effort required to record and document comparable information on all species of breeding birds in Britain and Ireland. The result of this huge survey - which concentrates on birds that breed in Britain and Ireland rather than those that simply migrate here for short periods - will be published this month as The New Atlas of Breeding Birds in Britain and Ireland: 1988-1991, the culmination of work by 20,000 amateur birdwatchers organised by the British Trust for Ornithology, the Irish Wildbird Conservancy and the Scottish Ornithologists' Club.

There is little doubt that the New Atlas will prove a seminal document, not just for assessing bird conservation needs but as an aid to identifying species-rich habitats, for land-use planning and educational purposes.

The publication of the New Atlas confirms that we know more about the status and distribution of the birds of the British Isles than of any other comparable area in the world. It builds on the methods used to produce its predecessor published in 1976, data for which was collected between 1968 and 1972.

Twenty years on, the distribution and population of many of Britain's and Ireland's breeding birds have changed markedly. Some of this change is natural but much is due to changes in land use, particularly in agriculture.

Take the attractive pink, blue and white jay. Its distribution map shows that the species is found throughout most of Wales and central, south and eastern England. But it is far more common in Kent, Sussex and Hampshire, than in the Welsh Marches, the Midlands and most of East Anglia where birdwatchers would probably have expected to find it in fairly equal numbers.

If jays reach their greatest concentrations where acorns are found in abundance, their high numbers in southern England - where there are oak trees aplenty - are explicable. But jays are apparently equally abundant in the East Anglian Breckland, where conifers outnumber oaks. So the acorn-jay linkage may not be universal.

Data also shows that the wren - with more than 7 million pairs - rather than the 'common' sparrow is the most abundant breeding bird. At the other extreme is the scarlet rosefinch. Just one nest was found in the four-year census period by a New Atlas fieldworker who discovered it in a gooseberry bush in coastal Sutherland. With its scarlet head and breast, this finch - spreading its range west from Scandinavia and northern Russia - may well establish itself in northern Scotland in future.

A total of 204 species were recorded as breeding in Britain and 137 in Ireland (a total of perhaps 85 million pairs) - in both cases two more species than were recorded between 1968 and 1972. Between the two censuses the great northern diver, black tern, snowy owl and hoopoe were lost as breeders. On the plus side, we gained the red-necked grebe, purple sandpiper, parrot crossbill and scarlet rosefinch.

A few pairs of whooper swans returned to breed in Scotland, while a pair of cranes set up home in East Anglia, the first to nest since the 17th century. And the magnificent sea eagle has been reintroduced to northwest Scotland after nearly 70 years.

Several other species, already abundant in some parts of Britain, have spread their wings. The siskin, a small greenish finch, was mainly confined to Scottish conifer forests two decades ago. But, as similar plantations further south have matured, this lively little bird has moved down with them. It is now breeding in many forested parts of Wales and the Lake District. Some have even settled in southern England. The siskin is now a common sight on many garden bird-tables in winter.

There are losers as well as winners among the Atlas's 427 pages. Many of these are farmland birds. Of 28 such birds - skylark and corn bunting for instance - 24 have contracting ranges, the result of changes in farming practices. Another is the barn owl. Its distribution map is decidedly patchy because of an overall 43 per cent reduction in range between 1968-72 and 1988- 91. This decline has been in progress for some time, the result of pesticide poisoning, loss of feeding habitat and, perhaps, the fact that there are fewer old buildings to provide breeding sites. But while there may be just 5,000 breeding pairs of these beautiful white owls left in Britain and Ireland, their future looks less bleak. With pesticide levels falling and much habitat loss stemmed - some even increasing - the fortunes of the barn owl could be improving.

After all, as the Atlas shows, sparrowhawks and peregrines, two birds of prey badly affected by pesticides in the Sixties and Seventies, have returned with gusto. Only the efforts of a highly committed - and entirely voluntary - workforce, willing to do the fieldwork for yet another Atlas in 20 or 30 years, will show us how the owls have fared.

'The New Atlas of Breeding Birds in Britain and Ireland: 1988-1991', compiled by David Wingfield Gibbons, James B Reid and Robert A Chapman, is published by T and A D Poyser, price pounds 40.

(Photographs omitted)